Absorbent articles such as sanitary napkins, panty liners, catamenials, incontinence inserts and diapers for adults or babies are commonly provided with an adhesive on their garment-facing surface to attach them during their usage period to a garment of the user, for example a pressure sensitive, hot melt, adhesive. These adhesives are typically covered with a release paper or strip prior to use.
More generally absorbent articles are provided with adhesive areas in order to combine the components that ultimately make up part or the whole of the absorbent article. In particular, multi-layer structures forming the topsheet, core or backsheet are often combined by adhesives called construction adhesives.
Typically these products are made by high-speed machinery. Current machinery includes equipment, such as spray guns or slot coaters that continuously or intermittently add the adhesive on the surface of an absorbent article. This needs to be done and can be done in a very fast manner, to ensure production at a very high speed.
One common drawback of all the above mentioned adhesive application processes is their inflexibility, inaccuracy relative to the shape of the adhesive to be applied.
For hollow drum screen-printing it is possible to create a pattern in the screen that would allow to create adhesive patterns. However, adhesive screen-printing is more restricted in providing an even, full surface adhesive coverage due to the maximum apertured dimensions and total open area of such a screen in respect to its stability.
Other proposed methods are for example described in WO 96/38113 and EP 745 433. These documents describe a method using a printing roll, which rotates through an adhesive bath and then contact the surface of an absorbent article, which passes on the top of the coating roll. The roll can contain a specific pattern of cavities and may be contacted with a scraper blade, so that excess material is scraped off and so that mainly the cavities are filled with the adhesive and thus, the pattern can be transferred to the absorbent article.
However, whilst the utilization of roll printing in principle also allows for the application of adhesives in patterns on surfaces, the process still has a number of problems associated with it. The print roll is continuously supplied with adhesive from an adhesive bath into which the roll is partially submerged and in which it is rotated. Naturally, the adhesive needs to be supplied in a large excess to allow the print roll to rotate through the adhesive bath and become coated with the adhesive. This of course requires a large amount of energy to be expended particularly in order to maintain the bath and adhesive at the required temperature. Moreover, the rotation of the roll within the bath causes the formation of air bubbles within the adhesive bath that results in the formation of foam. The foam is transferred to the roll and thereby results in the uneven distribution of the adhesive on the roll and consequently onto the substrate, even after scraping. Furthermore, the foam also collects on the scraper itself and is not readily removed there from whilst the process is operational.
Yet another problem with such a roll printing process is that the amount and distribution of adhesive that is deposited from the print roll onto the substrate is extremely difficult to control, resulting in a highly inefficient process. Also, the amount of stringing (i.e. fiberisation) during the transfer of the adhesive from the roll to the substrate surface is very large in this process. This results in an irregular application of the adhesive to the surface, in addition to contamination of the adhesive pattern itself.
As alternative process, WO 00/07533 suggest to replace the adhesive bath with a spraying tool or slot coater, positioned at the right or left hand side of the roll, which continuously applies an amount of adhesive onto a gravure printing roll with cavities, such that the cavities are filled to a certain extent only, which is then pressed against an absorbent article above the roll. A scraper blade may also be provided which scrapes off any excess adhesive.
Even in this alternative method, it has been found that it is difficult to apply sticky, stringy, viscous adhesives with precision, such that all cavities will contain the required amount of adhesive (e.g. if the volume of all cavities is the same, such that each cavity contains an equal amount of adhesive). This is in particularly the case when the process is performed at a high speed, such as normally necessary in economically feasible production processes, e.g. of more than 20 m/min, or even more than 100 m/min or even more than 150 m/min.
Furthermore, these known methods are such that the adhesive typically has to be heated to very high temperatures to be able to spray it, and that the temperature of the adhesive and the roll (or the difference between these temperatures) is difficult to control. Also whilst slot coaters can apply the adhesive very finely, the applied adhesive tends to clump together, seeking to minimize surface area. Thus an uneven application is obtained in practice. Furthermore, the adhesive applied with slot coaters tends to fly off the rotating gravure printing roll after application, especially when the adhesive clumps together and/or when they are very hot and more viscous.
Hence, there still exists a need to provide an improved (continuous) high speed process to apply such materials to articles, which overcomes the problem of the known processes as discussed above, and thus provides a more accurate and efficient way to apply materials including adhesives onto articles, typically in a shaped designs.
The inventors found that this is achieved by applying active materials, such as adhesives in a different manner. The active material is applied to the surface of a first tool (preferably a roll and typically a gravure printing roll with a pattern of cavities) by a coater unit having a multitude of applicators (extruders), which deposit a multitude of beads on the surface of the roll. The point of application of the coater is typically positioned above the first roll. Then preferably, a coating blade is pressed against the roll with a specific angle, to push the adhesive into the cavities. The coater blade is preferably positioned between the lowest point of the roll and the point where the adhesive is applied to the article. Then, the adhesive on the roll is brought in contact with the absorbent article, supported on a second tool, (preferably a roll, preferably being cooled and having a certain shore value of hardness).
Unlike the prior art processes, the process of the invention is such that the beads are formed in a precise way by the coater and remain as beads while on the surface; subsequently, they can be spread out to coat the surface evenly, for example by the preferred coater blade, as mentioned above. Also, the beads may be applied at lower temperatures than in prior art processes, using for example slot coaters. Also, because the beads are colder, they are stronger and thus do not fly off the surface after application. The process provides a much more accurate application of the active material onto the article: if the first surface is even, without cavities, a much more uniform and even application of the active material on the articles is achieved, compared to the prior art; if the surface has cavities which all have the same volume and serve to apply the adhesive in a dotted pattern onto the article, the dots have about the same size and about the same amount (weight) of adhesive. Thus, the articles have a more uniformly applied coating of the active material, either in the form of a uniform layer, or in the form of dots, which have uniformity in the amount of active per dot.
Furthermore, the process of the invention results in a significantly reduced level of contamination by stringing of the viscous material, i.e. due to the nature of the process, stringing of the material during application can be about avoided. This thus also helps to ensure that the adhesive or other active material is applied exactly as required, e.g. as a completely evenly applied layer, or in a very specific pattern, without built-up of string contamination.
The articles obtained by this process thus have a much more uniformly applied layer or (dot) pattern of the material, such as the adhesive, compared to the absorbent articles described or obtained in the prior art.